(a) Field of the Invention
The present invention relates to a printed circuit board constituting a part of an electronic device, more in particular, to the printed circuit board for suppressing unnecessary electromagnetic waves radiated therefrom.
(b) Description of the Related Art
With the demand of high speed operation and high integration of electronic devices, unnecessary electromagnetic wave radiation occurs from a device of which a function is not to radiate the electromagnetic waves, to raise public concern. Therefore, the unnecessary electromagnetic waves, especially those of 30 MHz to 1 GHz, must be legally controlled. Manufacturers of the electronic devices are requested to design and manufacture articles satisfying this legal standard.
In order to suppress the unnecessary electromagnetic wave radiation from the electronic devices, it is most effective that the radiation is suppressed at a printed circuit board in the device. Conventionally, a number of printed circuit boards have been devised having a means for radiation-control.
Examples of these printed circuit boards include that described in JP-A-06(1994)-244562, a circuit board having a low EMI structure disclosed in JP-A-09(1997)-205290, a low EMI multi-layered circuit board described in JP-A-09(1997)-283974, and electronic devices employing these printed circuit boards.
The characteristic features of these prior arts will be described referring to FIGS. 1 to 3.
In the printed circuit board shown in FIG. 1 which is disclosed in JP-A-06(1994)-244562, a part 131 of a power source layer 133 is separated from the remaining part, and is located on a substrate 134 near a ground layer 132. The power source layer 133 and the separated part (power source layer) 131 are connected by a coupling means 135 to increase an electrostatic capacity between the separated power source layer and the ground layer.
In the circuit board having the low EMI structure shown in FIG. 2 which is described in JP-A-09(1997)-205290, a printed circuit board 150 having a power source layer 151 on one surface and a ground layer 152 on the other surface is illustrated. Minute conductive patterns 154, 155 and minute conductive patterns 153, 156 are alternately disposed on the peripheries of the one layer and of the other layer, respectively. Every other conductive pattern 154 disposed on the periphery of the power source layer 151 is connected with the ground layer 152, and the other every other conductive pattern 155 is connected with the power source layer 151. The every other conductive pattern 153 disposed on the periphery of the ground layer 152 opposing to the above conductive patterns 154 is connected with the power source layer 151, and the every other conductive pattern 156 opposing to the above conductive patterns 155 is connected with the ground layer 152.
In the low EMI multi-layered circuit board shown in FIG. 3 which is described in JP-A-09(1997)-283974, a power source layer 162 and a first ground layer 163 form a capacity C1 interposing a dielectric layer 166, the power source layer 162 and a second ground layer 164 form a capacity C2 interposing a dielectric layer 167, and a resistor 165 is connected between the first ground layer 163 and the second ground layer 164.
In the circuit boards shown in FIGS. 1 to 3, all the circuit boards have a function for suppressing the variation of the power source voltage between the power source layer 162 and the first ground layer 163 both of which are a source of radiation. However, even if the power source layer and a part of the ground planar layer are so disposed that they are close to each other in the printed circuit board shown in FIG. 1, the increase of the electrostatic capacity obtained thereby is extremely small and the sufficient suppression of the power source voltage variation cannot be expected. In the circuit board 150 shown in FIG. 2, the polarities of voltages generated between the conductive patterns 154 and 153 and between the adjacent conductive patterns 155 and 156 are reversed to each other to have a function of implementing the radiation suppression by means of compensating the electric fields at the ends of the circuit board. Since, however, the voltage variation itself between the power source layer 151 and the ground layer 152 is unchanged, the unnecessary electromagnetic wave radiation from these layers 151 and 152 cannot be suppressed. While, in the multi-layered circuit board 161 shown in FIG. 3, the radiation of the unnecessary electromagnetic waves due to the voltage variation between the power source and the ground, and the malfunction of the device can be suppressed, the additional ground layer 164 and the second dielectric layer 167 are required in addition to the ordinary ground layer 163 to make its structure more complicated and to increase the cost.
All the above prior arts unavoidably require large alterations to the board structures. In order to apply the structural alteration to a printed circuit board already supplied as a product, redesign of the circuit board is required from the first step.
On the other hand, in order to suppress the variation of the power source voltage of IC or LSI, a capacitor having a large capacity is conventionally located between a power supply terminal and a ground terminal of IC or LSI. This conventional method utilizes a principle that variation of a power source voltage due to a switching of IC is reduced by making an impedance between a power supply terminal 171 and a ground terminal 172 lower than that of a capacitor ZC as shown in FIG. 4.
However, in a frequency band between 30 MHz and 1 GHz in which the radiation of the unnecessary electromagnetic waves raises the public concern (controlled frequency band), the capacitor element ZC shown in FIG. 4 cannot be regarded as a mere capacitor because of its parasitic inductance. A source line and other capacitors connected to IC are connected between the power supply terminals and the ground terminal of IC in addition to the capacitor element ZC, and the influence of these elements cannot be negligible.
A power supply circuit having a capacitor ZC1, and a source line and a capacitor ZC2 connected thereto is shown in FIG. 5. In order to reduce variation of a power source voltage in this circuit, an input impedance .vertline.Zin .vertline. observed from IC toward the power supply circuit must be reduced as much as possible. If, for example, a serial circuit including a source line having a length of 200 mm, its characteristic impedance of 50 ohms, its wavelength reduction rate of 0.5 and the capacitors ZC1 and ZC2 having 0.04 ohms, 0.7 nH and 0.1 .mu.F which are close to those of an actual capacity product is assumed to exist, its input impedance .vertline.Zin .vertline. becomes that shown as a solid line in FIG. 6. A broken line in FIG. 6 is a frequency characteristic of the absolute value .vertline.ZC .vertline. of an impedance of the single capacitor ZC. The impedances .vertline.Zin .vertline. and .vertline.ZC .vertline. function as capacitive resistances of which absolute values decrease proportional to wavelengths until about 17 MHz. Since the influence of the parasitic inductance of the capacitor becomes dominant over the above wavelength, the impedances function as an induced reactance in which the absolute value of the impedance increases proportional to the wavelength. Moreover, .vertline.Zin .vertline. becomes large at specified frequencies by means of the influence of the source line and ZC2. The variation of the power source voltage becomes large when a higher harmonic of an operating wavelength of IC is consistent with a wavelength of the source line and ZC2.
The reason why .vertline.Zin .vertline. becomes larger is occurrence of parallel resonance of the power supply circuit including the capacitor ZC1, the source line and the capacitor ZC2. Therefore, a large current flows in the source line at a frequency at which the input impedance .vertline.Zin .vertline. becomes large, and electromagnetic waves radiated therefrom also become large.
A proposal for overcoming the above problem is described in JP-A-10(1998)-112574. In this proposal, four first capacitors for connecting a power source layer and a ground layer disposed on the periphery of a printed circuit board and a second capacitor disposed near IC supplying a transitional current to IC are connected between the power source layer and the ground layer of the .circuit board. However, in this structure, a plurality of the first capacitors must be mounted along the periphery of the circuit board, and when a large circuit board is to be employed, the number of the capacitors required becomes large.
JP-B-07(1995)-46748 also solves a similar problem. In the printed circuit board described therein, a .pi.-type filter consisting of a capacitor, ferrite beads and a capacitor is employed. However, the .pi.-type filter containing the ferrite beads is expensive to invite the cost increase of the printed circuit board.